This invention pertains to implantable medical devices such as cardiac pacemakers and implantable cardioverter/defibrillators. In particular, the invention relates to a system and method for receiving telemetry data from such devices.
Implantable medical devices, including cardiac rhythm management devices such as pacemakers and implantable cardioverter/defibrillators, typically have the capability to communicate data with a device called an external programmer via a radio-frequency telemetry link. One use of such an external programmer is to program the operating parameters of an implanted medical device. For example, the pacing mode and other operating characteristics of a pacemaker are typically modified after implantation in this manner. Modem implantable devices also include the capability for bidirectional communication so that information can be transmitted to the programmer from the implanted device. Among the data which may typically be telemetered from an implantable device are various operating parameters and physiological data, the latter either collected in real-time or stored from previous monitoring operations.
Noise refers to any unwanted signal that interferes with the transmission and processing of data signals in a communications system. Such noise may arise from sources either internal or external to the system. Because of limited energy storage capability, implantable medical devices must necessarily transmit their data with a low signal energy, making the transmissions very susceptible to interference from noise. This means that an external programmer can only be satisfactorily used to receive data in relatively noise-free environments. Because of the widespread nature of electromagnetic noise sources, such a constraint may not only be inconvenient to the patient and clinician, but could also be hazardous in an emergency situation. Both broadband and narrowband noise sources contribute to the problem, with modern CRT monitors being a particularly common source of narrowband noise.
It is an objective of the present invention to provide a system and method for receiving telemetry data from an implantable medical device with an improved capability for operating in noisy environments. In accordance with the invention, a telemetry data receiving system receives a radio-frequency signal transmitted from an implantable medical device with an antenna suitable for positioning in proximity to the device. In one embodiment, the transmitted signal is a carrier waveform modulated with digitally encoded data in the form of transmit pulses. The received signal is digitized into input signal samples that are input to a matched filter having filter coefficients that correspond to a transmit pulse. A pulse detector compares output values of the matched filter with an adaptive pulse threshold value in order to detect the presence of transmit pulses within the input signal samples. Adaptation is implemented by a threshold adjustment routine which adjusts the pulse threshold value in accordance with measured peak values of both noise and the transmitted signal.
In a particular embodiment, narrowband noise is removed from the input signal samples with a series of notch filters having center notch frequencies generated adaptively so that the notch frequencies match the frequency peaks of a detected noise spectrum. The noise spectrum is detected by first computing a power spectrum of the input signal and then subtracting from it a template spectrum corresponding to an expected input signal without noise. A template spectrum is computed from a representative input signal generated under noise-free conditions so that when it is subtracted from the input signal spectrum, the result approximates the power spectrum of the narrowband noise alone. In order to produce a detected noise spectrum that most closely approximates the true noise spectrum, the template spectrum is scaled by a factor that reduces the total power in the detected noise spectrum to a minimal value. The frequency peaks in the detected noise spectrum are then identified and used to synthesize filters with corresponding notch frequencies to remove the noise from the input signal.